Antimicrobial Activity of Selected Medicinal Plants Used in Lithuanian Traditional Medicine

Introduction Although some plants have been widely used for curing infectious diseases for centuries, it is only recently that scientists have discovered that one of the reasons certain medicinal plants (MPs), including those traditionally used in Lithuania (thyme [1], peppermint [2], sage [3], clove [4], parsley [5], and fireweed [6]), have healing properties is the antimicrobial activity (AA) of their extracts. However, researchers often study only one MP and use different methods, and this makes it difficult to compare the AA of different MPs. Therefore, it is relevant to investigate the AA of a larger number of MPs in one study. Aim To determine the AA of selected MPs depending on the concentration of the MP extract and the species of microorganism exposed. Methods The dilution method was chosen in this study as it can be used to not only qualitatively but also quantitatively investigate the AA by determining the minimum inhibitory concentration (MIC) of MP extracts [7]. Reference cultures of 9 bacteria (Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecalis, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Proteus vulgaris, Bacillus cereus, Bacillus subtilis) and the fungus Candida albicans were used. The selected MPs were each reduced to powder, mixed with 100 ml 70 % ethanol and extracted in an ultrasound bath for 30 min at 40 ºC. Each liquid extract was then filtered into flasks and stored at 50 ºC in a water bath for 3 days to evaporate the ethanol and produce the viscous extracts. The qualitative study was performed by mixing the liquid extracts with agar in Petri dishes and observing if there was any growth of microorganisms. Then, the quantitative study was conducted by testing the viscous extracts in the same way but at different concentrations and determining the MIC. Results In the qualitative study, parsley was the only MP that failed to exhibit AA against all tested microorganisms, inhibiting the growth of only B. cereus and B. subtilis, and the quantitative study revealed that the MIC for both bacteria was 27,39 mg/ml. The viscous extract of sage had the strongest AA against most Gram-positive bacteria (S. aureus, S. epidermidis, B. subtilis (MIC 1,32 mg/ml) E. faecalis (MIC 5,13 mg/ml), P. vulgaris (MIC 3,88 mg/ml)). E. coli (MIC 16,67 mg/ml), K. pneumoniae and B. cereus (MIC 2,91 mg/ml) were the most sensitive to the thyme extract, P. aeruginosa – to the fireweed extract (MIC 14,76 mg/ml). C. albicans was sensitive only to the viscous extracts of sage, clove and fireweed, clove being the most effective at inhibiting its growth (MIC 6,44 mg/ml). Conclusions The strongest AA against C. albicans was observed with the clove extract. The viscous extracts of sage, clove and fireweed had the broadest spectrum AA (effectively inhibited the growth of both the tested bacteria and C. albicans), the sage extract showing the strongest AA against most Gram-positive bacteria. The parsley extract exhibited the weakest AA.